iPhone X auto-locks in 10s when viewing from reclined position

This is fun: iPhone X shuts off the screen when viewing from bed in 10 seconds. Has no one else seen this? I can reliably repro it on multiple phones, and it makes using it super frustrating to use from a reclined position.

Steps:

1. Set auto-lock to 30 seconds (Settings > Display & Brightness).

2. In a dimly lit room, look up and hold the iPhone X up over your head facing down towards your face.

3. Open a page in Safari, look at it, and hold it steady. (This is important.. any movement will keep it awake.)

Result: Screen dims in ~7 seconds and shuts off completely within 10.

Expected result: The phone should detect your attention and stay unlocked and lit. Even if it doesn't detect attention, dimming may be acceptable but it should not lock prior to the auto-lock setting.

Workaround: Set auto-lock to Never or 5 minutes.

Increasing the auto-lock setting will delay this, but notably it still happens much sooner than the setting value (e.g. 1m auto-lock = locks in ~15s). So setting it to 5 minutes and it probably won't happen before you move or interact with the screen again. Or set it to Never and perhaps it won't happen at all.

Attention awareness setting has no effect.

iOS: 11.2